Recents in create, uniform item layouts.

Show recent directories in a separate fragment, and save as serialized
versions of new DirectoryStack. Cleaner behavior around recents and
search, instead of treating them as pseudo-Documents.

More uniform item layouts between list and grid, including both date
and size information, and originating storage root in recents. Avoid
clashing directory loaders by assigning unique numbers. Promote
list/grid switching up into activity.

Change-Id: I9a93460b896067ca036d7e772eeabde31face2e1